The protection of desirable plants and their produce from fungal and bacterial pathogen infection has traditionally required preventative applications of fungicidal or bactericidal agents. Fungicidal and bactericidal compounds have long been used to increase yields and extend agricultural production capabilities into new areas. They have also been extremely important tools for ameliorating season-to-season differences in yield and quality caused by weather-driven variations in disease pressure.
The future role of fungicides and bactericides in agriculture is increasingly threatened by several factors including; the development of pest resistance, increasing concerns about food safety, and environmental accumulation of toxic compounds. As older fungicides and bactericides are removed from the market due to regulatory changes, and new fungicides and bactericides are becoming increasingly expensive to register, there is an increasing need to find ways to more wisely use the remaining, safest fungicides. This is particularly true for the many crop/disease combinations which do not represent large enough markets to pay for the cost of new compound registration. Wiser fungicide and bactericide use will include ways to reduce application rates (and thus potential residues), finding ways to extend registrations to new crops, and identifying new fungicidal and bactericidal compositions and treatments to combat the development of pest resistance.
Chemical fungicides and bactericides have provided an effective method of control; however, the public has become concerned about the amount of residual chemicals which might be found in food, ground water and the environment. Stringent new restrictions on the use of chemicals and the elimination of some effective pesticides from the market place could limit economical and effective options for controlling fungi and bacteria.
It is well recognized by those skilled in this art that there is a clear distinction between preventative microbicidal (fungicidal and bactericidal) activity and curative activity. Compositions and methods which may be effective to prevent microbial growth may have very little or no impact on established infections. Of course, it is often desirable to prevent infections altogether, however, this is not always possible and there is a great need for compositions which have the unique ability to arrest the growth of established infections. This is particularly true in the control of infections which become established on agricultural products after harvest.
Curative fungicidal activity has been observed when some biological agents are used for disease control (e.g. strain of Bacillus subtilis) and this activity can usually be attributed to the production of antibiotic compounds by the biocontrol organism. Because expensive toxicological screening and residue/metabolite monitoring may be required for such an antibiotic, the normal registration-cost advantage of these nonchemical agents is diminished. Biological control agents which do not make antibiotics would be much easier to register, but they tend to have only preventive control.
The commercialization of disease biocontrol agents has also been hampered by inconsistent field performance. Organisms which show biocontrol potential in laboratory and greenhouse experiments often fail to compete with the existing microflora when applied outdoors and are thus unable to express their biocontrol potential, regardless of mode of action. Specifically there is a need for disease control methods which are more compatible with the need for affordable and effective disease control, a high degree of food safety, and minimal environmental impact.
One example of the need to control post-harvest spoilage of agriculture products pertains to green and blue molds of citrus fruits caused by Penicillium digitatum and P. italicum. These molds cause severe damage during storage and shipping. The existing fresh-market industry relies completely on a combination of several chemical treatments to deliver sound fruit to distant markets over substantial periods of time without excessive damage caused by these molds. Unfortunately, there are increasing concerns about the safety of the chemicals currently used to control these fungal pathogens. Also, there are increasing problems with fungal strains with resistance to the most effective compounds.
In another example, powdery mildew of grapes caused by Uncinula necator can cause severe damage even in dry areas such as California. Traditionally this disease was controlled with applications of elemental sulfur, but this necessitates frequent, high volume applications of an irritating material. The introduction of egosterol biosynthesis inhibiting fungicides (primarily triazoles) greatly simplifies control, but also selects for tolerant strains. Some of these compounds are also known to have potential teratogenic effects and very long soil residuals. In these and other examples, alternative control methods are in great demand--particularly methods which are safer or more environmentally benign.
Fatty acids are a class of natural compounds which occur abundantly in nature and which have interesting and valuable biological activities. The by vitro activity of fatty acids against many medically important fungi and bacteria is well known; however, their in vivo antifungal activity is often very limited and it is difficult to predict on the basis of in vitro experiments. There is a much smaller body of literature concerning the activity of fatty acids and their derivatives against pathogens on agricultural crops. Ahmed et al. (Ahmed, S. M., F. Ahmad, S. M. Osman [1985] JAOCS 62:1578-1580) report in vitro inhibition of radial growth of several fungal genera with plant pathogenic representatives. Recently there has been an expanding use of "insecticidal soaps" in agriculture which are salts of certain fatty acids. This has resulted in a few observations of impact on fungal disease. For instance, Chase et al. (Chase, A. R., L. S. Osborne [1983] Plant Disease 67:1021-1023) observed that applications of an 18:1 fatty acid salt "insecticidal soap" gave moderate preventive control of two foliage plant diseases and actually exacerbated two other diseases. In U.S. Pat. No. 3,983,214, Misato et al. claim a fungicidal composition containing a sucrose fatty acid ester. Misato et al. emphasize the preventative activity of their composition. Similarly, in U.S. Pat. No. 4,771,571, Obrero et al. describe a method of preventing infections of pineapple by treating the fruit, while on the tree, with a surfactant. In U.S. Pat. No. 4,002,775, Kabara et al. claim microbicidal food additives comprising 1 or 2-mono-laurin polyol ester. Kabara's work is also described in: Chapter 14 of Ecology and Metabolism of Plant Lipids, American Chemical Society (1987); "Fatty Acids and Derivatives as Antimicrobial Agents," In: Antimicrobial Agents and Chemotherapy, American Society for Microbiology (1972), pp. 23-28; "Antimicrobial Agents Derived from Fatty Acids," (1984) JAOCS 61(2):397-403; and "Antimicrobial Lipids: Natural and Synthetic Fatty Acids and Monoglycerides," Lipids 12(9):753-759. Also, the use of fatty acid esters and alcohols for the control of powdery mildew on apple buds (Frick, E. L., R. T. Burchill [1972] Plant Disease Reporter 56:770-772), but this work did not touch on fatty acids themselves or on their salts. Most in vitro tests for antimicrobial activity involve monitoring the germination and growth of pathogen propagules in a liquid or solid format in which there is exposure to the chemical agent. These assays are directly analogous to preventive applications in an agricultural setting--applications which are made prior to the time when the pathogen initiates an infection. The primary screening process for synthetic chemicals in industrial settings is almost exclusively based on in vitro and preventive in vivo testing. Thus, compounds without significant preventive activity are rejected. There are no reports of fatty acids acting in a curative mode (applied after fungal infection is established).